Ungrounded 1,000VDC 18KW string inverters

[Neal30]

I have been looking at an 1,000VDC Ungrounded string inverter. The manufacturer states that there is no fusing required on the DC side of this inverter. The reasoning behind this is that the inverter has three separate MPPT channels internally. Each channel only accepts two strings. The NEC code that they reference on OCPD is the one about no more than two strings and the sum of all potential currents not exceeding the amp rating of the conductor. It makes me a little weary not having any DC fuses, especially in this case where both the positive and negative would be fused. The manufacturer worked hard at explaining the logic and the interpretation, but from a risk reward standpoint, I would like to hear how some other industry professionals would weigh in here. We are talking about dropping 43 of these inverters in a field in a utility style project.
My thought is that even though the MPPT channels are separated the wires would all be in close proximity inside the inverters termination housing.

If none of the strings have a relationship to the ground anywhere, and they are not more than two parallel circuits inside the inverter, that could potentially impose more current due to the seperate mppt channels, than the instance where more than two circuits could be paralleling and have a combined current would be Nell? How are the MPPT channels isolated would be a question. It is very interesting, and I kind of rambled there for a minute but it seems that I answered my own question. I still would like to here some thoughts, if anyone is interested. Thanks!

 

[GoldDigger]

It is very interesting, and I kind of rambled there for a minute but it seems that I answered my own question. I still would like to here some thoughts, if anyone is interested. Thanks!

As far as fuses intended to protect the individual panel strings against reverse overcurrent caused by a failure, the fuses do not seem to be required, no.
Now does the inverter REQUIRE that the string be ungrounded, and what will happen to an MPPT channel which sees a ground-referenced DC supply as the result of a fault to ground? If the inverter will not be damaged by that, then there is no need for fuses on that ground. (The inverter should have ground detection as part of its input circuit anyway, but the activation of that circuit may leave a ground-referenced 1000 volts somewhere on the DC side of the system.)

Where there may still be a justification for fusing is the amount of power available at a line to line fault in such a system. For most situations the current limited output of a PV panel results in no energy dissipation at a bolted fault, but for a 1000 volt string and a less than solid fault, there is a lot of energy available. But no single fuse location could handle all line-to-line fault scenarios.

 

[jaggedben]

My thought is that even though the MPPT channels are separated the wires would all be in close proximity inside the inverters termination housing.

By this logic, putting fuses for multiple circuits in the same housing is useless. You could make the same criticism about every solar combiner product on the market, many of which handle far more than your six source circuits. That makes me think you're going a bit overboard in your worries.

The reality is that with your inverters it would take multiple faults in combination to create a fault situation where a fuse would even be helpful. One or more of those faults should be detected by the inverters GFDI before an actual overcurrent situation occurs. These inverters are really truly different from paralleled circuits.

How are the MPPT channels isolated would be a question.

I think it's doubtful that an overcurrent could be conducted through the MPPT power electronics in reverse. But even if this were possible, I think it's likely that the inverter includes a contact for each MPPT that would be opened in case of a GFDI detected fault or other error. If you happen to be using Power-One inverters, their block diagrams show these contacts. Hopefully that gives you some peace of mind.

 

[dereckbc]

I think what you have is a transformerless inverter allowed since 2005 code cycle 690.35 Is that what you have? If so both polarities are required to have fuses because the system conductors are not grounded and float. However GFCI is required, and most of the transformerless Inverters have it buit-In GFCI to comply with 690.5 to save on installation and material cost. It will also require you to use PV rated cable of 1000 volts or higher rather than the standard USE-2 600 volt cable to comply with 690.35. High voltage PV cable is double insulated.

With a grounded PV array, the grounded conductor (typically DC minus) cannot be broken or switched. The ungrounded conductor requires over current protection according to NEC 690.9 when more than two strings are connected in parallel. In transformerless systems, both DC plus and DC minus are floating ungrounded, thus over current protection is required for both conductors when multiple DC source circuits are connected in parallel.

 

[GoldDigger]

However GFCI is required, and most of the transformerless Inverters have it buit-In GFCI to comply with 690.5 to save on installation and material cost.

I though that what was required here was ground detection (same as for an ungrounded delta AC service), not GFCI? Or are both actually required?

 

[jaggedben]

I think what you have is a transformerless inverter allowed since 2005 code cycle 690.35 Is that what you have? If so both polarities are required to have fuses because the system conductors are not grounded and float.

I think the OP's question really revolves around the issue of whether multiple MPPT inputs on an inverter are to be considered paralleled circuits or not. (I would say they are not.) The question is equally relevant for systems with or without a grounded conductor.

The ungrounded conductor requires over current protection according to NEC 690.9 when more than two strings are connected in parallel. In transformerless systems, both DC plus and DC minus are floating ungrounded, thus over current protection is required for both conductors when multiple DC source circuits are connected in parallel.

Although the bits in red usually work as rule of thumb, the actual exceptions in 690.9(A) are more specifically technical. I do not believe that the OPs situation requires OCPD, although he could be leaving out details that would change that.

However GFCI is required,...

I though that what was required here was ground detection (same as for an ungrounded delta AC service), not GFCI? Or are both actually required?

690.5 contains the requirements for solar systems, which are termed Ground Fault Detection and Interruption in 690.5(A), from which we get GFDI. The requirements are not technically the same as GFCI as defined in Article 100, and although the general concept is similar, the actual devices can be very different. It's better to say GFDI when referring to solar so as not to cause confusion.

 

[shortcircuit2]

I though that what was required here was ground detection (same as for an ungrounded delta AC service), not GFCI? Or are both actually required?

The requirement for ground detectors 250.21(B) is for AC systems...